1. Field of Invention
The present invention is related to a micro-electro-mechanical-system (MEMS) lithography mask, in particular to one that improves tungsten deposition topography, and a method for improving tungsten deposition topography.
2. Description of Related Art
A MEMS typically contains, a MEMS device and a microelectronic circuit which need to be integrated with each other to constitute an integrated MEMS chip. Such integration is an important concern in the manufacturing process of a MEMS chip.
FIG. 1 shows a partial pattern 100 of a prior art MEMS lithography mask for defining a tungsten layer; what is shown is a partial pattern of the MEMS device, while the pattern of the microelectronic circuit is not shown. Tungsten is an important material often used for making plugs in the interconnection of a microelectronic circuit; a typical process for forming such tungsten plugs is damascene process. A damascene process includes steps as follows: first, etching trenches in a silicon dioxide layer according to a desired pattern; second, depositing tungsten in the trenches by, e.g., chemical vapor deposition; and last, polishing the surface by chemical mechanical polish to prepare for later deposition of metal wire layer. In the same time as the tungsten being deposited in the microelectronic circuit area, a tungsten layer is also formed in the MEMS device area. This tungsten layer in the MEMS device area is to constitute a mechanical structure, for example, to constitute a part of a MEMS device, such as a movable component, a fixed component, a spring, or an anchor, etc. in combination with an upper or lower metal layer.
Please refer to FIGS. 2, 2A and 2B, which show the 3-dimensional structure of the MEMS device. A movable electrode 30 (FIG. 2A) and fixed electrodes 40 (FIG. 2B) are formed on a substrate 20, wherein the fixed electrodes 40 are fixed to the substrate 20 by one or more structure layers 12 below. When the movable electrode 30 move in the direction shown in FIG. 2A, the gaps between the movable electrode 30 and the fixed electrodes 40 are changed so that the capacitance is changed accordingly; a displacement can thus be measured. In this example, the mask shown in FIG. 1 is used to define the structure of the aluminum layer 10 and the tungsten layer 11.
Referring back to FIG. 1, in the mask pattern 100 which defines the tungsten layer, often there are two or more sections forming a conjunction with each other. For example, in the cross-shaped pattern 110 at the left of FIG. 1, a conjunction is formed in the dotted circle. If the cross-shaped pattern 110 in FIG. 1 is used to define a trench for depositing tungsten, the topography as shown in FIG. 3A might be formed, and the cross-section view along the cross-section line AB would appear as shown in FIG. 3B. As one can see from FIG. 3B, the center of the conjunction is not fully filled with tungsten because it is far from the edges of the sections. Therefore, there is a cavity at the center of the conjunction. And if another metal layer 10, such as aluminum or copper layer, is deposited thereon, the metal layer will also follow the topography and form a smaller cavity at the center. Because the surface topography is not even, the stress between different metal layers increases accordingly, which may cause malfunctions in the MEMS such as due to errors in the relationships between the movable electrodes and fixed electrodes.
In the prior art mentioned above, the damascene process of tungsten generates cavities in the topography in the MEMS device area and may affect the functionality of the MEMS device; this is disadvantageous. In view of such drawback, the present invention proposes a MEMS lithography mask with improved tungsten deposition topography and a corresponding method to overcome it.